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Encyclopedia > Field effect transistor
Large power N-channel field effect transistor
Large power N-channel field effect transistor

The field-effect transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a 'channel' in a semiconductor material. FETs are sometimes used as voltage-controlled resistors. The concepts related to the field effect transistor predated those of the bipolar junction transistor (BJT). Nevertheless, FETs were implemented only after BJTs due to the simplicity of manufacturing BJTs over FETs at the time. Image File history File links P45N02LD.jpg Large power N-channel field effect transistor P45N02LD (better described in [1]). Photo by Audrius Meskauskas Audriusa 12:57, 22 January 2006 (UTC) File links The following pages link to this file: Field effect transistor ... Image File history File links P45N02LD.jpg Large power N-channel field effect transistor P45N02LD (better described in [1]). Photo by Audrius Meskauskas Audriusa 12:57, 22 January 2006 (UTC) File links The following pages link to this file: Field effect transistor ... Assorted transistors A transistor is a semiconductor device that uses a small amount of voltage or electrical current to control a larger change in voltage or current. ... It has been suggested that optical field be merged into this article or section. ... Electrical conductivity is a measure of a materials ability to conduct an electric current. ... A semiconductor is a solid whose electrical conductivity can be controlled over a wide range, either permanently or dynamically. ... Resistor symbols (US and Japan) Resistor symbols (Europe, IEC) A pack of resistors A resistor is a two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohms law. ... A bipolar junction transistor (BJT) is a type of transistor. ...

Contents

Terminals

All FETs except J-FETs have four terminals, which are known as the gate, drain, source and body/base/bulk. Compare these to the terms used for BJTs: base, collector and emitter. BJTs and J-FETs have no body. It is common in large FETs to connect the body and source internally to simplify design. In most applications one would connect the source to the body anyway. The voltage applied between the gate and source terminals modulates the current between the source and drain terminals. A difference between the voltages of the source and body will change the threshold voltage. This is known as the body effect and is used primarily in digital circuits, although it is taken into account in high precision analog circuits. There are two 'modes' of FET: enhancement, in which a voltage applied to the gate increases the current flow from source to drain; and depletion, in which a voltage applied decreases the current flow from source to drain. Thus enhancement FETs are normally off, whereas depletion FETs are normally on. Depletion Region of an NMOS The threshold voltage of a MOSFET is usually defined as the gate voltage where a depletion region forms in the substrate (body) of the transistor. ...


Composition

Most FETs are made with conventional bulk semiconductor processing techniques, using the single crystal semiconductor wafer as the active region, or channel. NASAs Glenn Research Center cleanroom. ... A single crystal is a crystalline solid in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample. ... A semiconductor is a solid whose electrical conductivity can be controlled over a wide range, either permanently or dynamically. ...


Types of field-effect transistors

Depletion type FETs under typical voltages. JFET, poly-silicon MOSFET, double gate MOSFET, metal gate MOSFET, MESFET.  depletion ,  electrons ,  holes ,  metal ,  insulator . Top=source, bottom=drain, left=gate, right=bulk. Voltages that lead to channel formation are not shown
Depletion type FETs under typical voltages. JFET, poly-silicon MOSFET, double gate MOSFET, metal gate MOSFET, MESFET.  depletion ,  electrons ,  holes ,  metal ,  insulator . Top=source, bottom=drain, left=gate, right=bulk. Voltages that lead to channel formation are not shown

The FET is simpler in concept than the bipolar transistor and can be constructed from a wide range of materials. The channel region of any FET is either doped to produce an N-type semiconductor, giving an "N-channel" device, or with a P-type to give a "P-channel" device. The doping determines the polarity of gate operation. The different types of field-effect transistors can be distinguished by the method of insulation between channel and gate: Image File history File links Download high resolution version (972x764, 48 KB) Summary Enhancement type FETs. ... Image File history File links Download high resolution version (972x764, 48 KB) Summary Enhancement type FETs. ...

  • The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) utilizes an insulator (typically SiO2).
  • The JFET (Junction Field-Effect Transistor) uses a p-n junction as the gate.
  • The MESFET (Metal-Semiconductor Field-Effect Transistor) substitutes the p-n-junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.
  • Using bandgap engineering in a ternary semiconductor like AlGaAs gives a HEMT (High Electron Mobility Transistor), also called an HFET (heterostructure FET). The fully depleted wide-band-gap material forms the isolation.
  • The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure formed by graded doping of the active region.

Among the more unusual body materials are amorphous silicon, polycrystalline silicon or other amorphous semiconductors in thin-film transistors or organic field effect transistors that are based on organic semiconductors and often apply organic gate insulators and electrodes. The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), is by far the most common field-effect transistor in both digital and analog circuits. ... R-phrases R42 R43 R49 S-phrases S22 S36 S37 S45 S53 Flash point non-flammable Supplementary data page Structure and properties n, εr, etc. ... Electric current flow from source to drain in a JFET is restricted when a voltage is applied to the gate. ... MESFET stands for Metal-Semiconductor Field Effect Transistor. ... A Schottky barrier is a metal-semiconductor junction which has rectifying characteristics, suitable for use as a diode. ... In solid state physics and related applied fields, the band gap is the energy difference between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. ... Aluminium gallium arsenide (also Aluminum gallium arsenide) (AlxGa1-xAs) is a semiconductor material with very nearly the same lattice constant as GaAs, but a larger bandgap. ... HEMT stands for High Electron Mobility Transistor, and is also called heterostructure FET (HFET). ... The modulated-doping field effect transistor or modulation-doped field effect transistor (MODFET) is a type of a field-effect transistor. ... A quantum well is a potential well that confines particles in one dimension, forcing them to occupy a planar region. ... Amorphous silicon (a-Si) is the non-crystalline allotropic form of silicon. ... Polycrystalline silicon or polysilicon or poly-Si or simply poly (in context) is a material consisting of multiple small silicon crystals, and has long been used as the conducting gate material in MOSFET and CMOS processing technologies. ... A thin film transistor (TFT) is a special kind of field effect transistor made by depositing thin films for the metallic contacts, semiconductor active layer, and dielectric layer. ... An Organic Field-Effect Transistor (OFET) is a class of thin film transistors based on organic semiconductors. ... An organic semiconductor is an organic compound that exhibits similar properties to inorganic semiconductors, specifically the presence of a hole and electron conduction layer and a band gap. ...


FET Operation

The shape of the conducting channel in a FET is altered when a potential difference is applied to the gate terminal (potential relative to either source or drain.) In an n-channel "depletion-mode" device, a negative gate voltage causes a depletion region to expand in size and encroach on the channel from the side, narrowing the channel. If the depletion region completely closes the channel, the resistance of the channel becomes very large, and the FET is effectively turned off. Positive gate voltage attracts electrons from the surrounding semiconductor next to the gate, forming a conductive channel. At low source-to-drain voltages, small changes to the gate voltage will alter the channel resistance. In this mode the FET operates like a variable resistor. This mode is not employed when amplification is needed. In semiconductor physics, the Depletion Zone or Depletion layer is a nonconductive region within a conductive, doped semiconductor material where the charge carriers have been swept away. ...


If a larger potential difference is applied between the source and drain terminals, this creates a significant current in the channel and produces a gradient of potential from source to drain. This also causes the shape of the depletion region to become asymmetrical–one end of the channel becomes narrow. If the potential difference is large enough, the depletion region begins to close the channel. The FET is said to be in saturation. Rather than entirely blocking the electrons from flowing from source to drain, electrons flow through the depletion region in a controlled manner. Any attempted increase of the drain-to-source voltage will lengthen the depletion region, increasing the channel resistance proportionally with the applied drain-to-source voltage which causes the value of drain current to remain relatively fixed. This mode of operation is called pinch-off. In this mode, the FET behaves as a constant-current source rather than as a resistor and can be used as a voltage amplifier. The value of gate voltage determines the value of the constant current in the channel. An ideal current source, I, driving a resistor, R, and creating a voltage V A current source is an electrical or electronic device that delivers or absorbs electric current. ...


An "enhancement-mode" device is of slightly more complex construction. Rather than only being one material, it is made of a three-piece sandwich: similar to either an npn or a pnp transistor with no base connection, but still with a gate. In this case, the device is "normally-off" since one of the two junctions will always be reverse biased. The npn device is called an n-channel device and the pnp device is called a p-channel device.


Uses

The most commonly used FET is the MOSFET. The CMOS (complementary-symmetry metal oxide semiconductor) process technology is the basis for modern digital integrated circuits. This process technology uses an arrangement where the (usually "enhancement-mode") p-channel MOSFET and n-channel MOSFET are connected in series such that when one is on, the other is off. In CMOS logic devices, the p-channel device pulls up the output and the n-channel device pulls down the output. The great advantage of CMOS circuits is that they allow no current to flow (ideally), except during the transition from one state to the other, which is very short. The gates are capacitive, and the charging and discharging of the gates each time a transistor switches states is the primary source of power usage in fast CMOS logic circuits. However as integrated circuits become smaller, parasitic resistances are becoming more power consumptive than switching capacitance. The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), is by far the most common field-effect transistor in both digital and analog circuits. ... Static CMOS Inverter Complementary-symmetry/metal-oxide semiconductor (CMOS) (see-moss, IPA:), is a major class of integrated circuits. ... A digital system is one that uses discrete values (often electrical voltages), especially those representable as binary numbers, or non-numeric symbols such as letters or icons, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (ie, as in an analog system). ... Integrated circuit showing memory blocks, logic and input/output pads around the periphery Microchips with a transparent window showing the integrated circuit inside. ... A fab is a factory for producing integrated circuits. ... Integrated circuit showing memory blocks, logic and input/output pads around the periphery Microchips with a transparent window showing the integrated circuit inside. ...


The fragile insulating layer of the MOSFET between the gate and channel makes it vulnerable to electrostatic damage during handling. This is not usually a problem after the device has been installed. To meet Wikipedias quality standards, this article or section may require cleanup. ...


FETs can switch signals of either polarity on the source or drain terminals, if their amplitude is significantly less than the gate swing, as the devices are typically symmetrical. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board, for example. In telecommunications, multiplexing (also muxing or MUXing) is the combining of two or more information channels onto a common transmission medium using hardware called a multiplexer or (MUX). ... BBC Local Radio Mark III radio mixing desk In professional audio, a mixing console, mixing desk (Brit. ...


The power MOSFET has a reverse-biased 'parasitic diode' shunting the conduction channel that has half the current capacity of the conduction channel. Sometimes this diode is used when driving inductive circuits, but in other cases it causes problems. Types of diodes closeup, showing germanium crystal In electronics, a diode is a component that restricts the direction of movement of charge carriers. ... Inductance (or electric inductance) is a measure of the amount of magnetic flux produced for a given electric current. ...


A more recent device for power control is the insulated-gate bipolar transistor, or IGBT. This has a control structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These have become quite popular in the 200-3000 V range of operation, as they overcome limitations of Power MOSFET in high voltage. Power MOSFETs are still the device of choice (and practically the only choice available) for low voltage (from less than 1 V to 200 V) applications. The Insulated Gate Bipolar Transistor combines the simple gate drive characteristics of the MOSFET with the high current and low saturation voltage capability of bipolar transistors by combining an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device. ...


See also

A FREDFET (sometimes, FredFET) is a Fast-Reverse Epitaxal Diode Field-Effect Transistor or . ...

External links


  Results from FactBites:
 
Field Effect Transistors (555 words)
The common transistor is called a junction transistor, and it was the key device which led to the solid state electronics revolution.
In application, the junction transistor has the disadvantage of a low input impedance because the base of the transistor is the signal input and the base-emitter diode is forward biased.
For an n-channel FET, the device is constructed from a bar of n-type material, with the shaded areas composed of a p-type material as a Gate.
  More results at FactBites »

 
 

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